I am slowly working on my aquascape, do a little each week. Hell, I'm retired so whats the hurry, right?! I mean, look at my picture over there on the left, does it look like I get in a hurry about anything?!!

My question is, is it safe to add the Contrasoil a few weeks before I plant and add water? Any reason why it just can't sit in the aquarium until I'm damned good and ready for the next step?

So long as the aquarium is dry there is no reason not to build your hardscape (soil, rocks, wood) however you like it. You can let it sit forever or 10 minutes or a week. It makes no difference.

Once you add water with an active substrate you definitely want to have plants in there because the soil will begin releasing LOTS of nutrients. You will need to be doing daily water changes for the first week etc and your plants will be helping out by soaking up everything they can handle as well. This will keep the algae under control.

Before water, doesn't matter. Take your time ;P